Rails find record with zero has_many records associated

Bah, found it here: https://stackoverflow.com/a/5570221/417872

City.includes(:photos).where(photos: { city_id: nil })

In Rails versions >= 5, to find all cities that have no photos, you can use left_outer_joins:

City.left_outer_joins(:photos).where(photos: {id: nil})

which will result in SQL like:

SELECT cities.*
FROM cities LEFT OUTER JOIN photos ON photos.city_id = city.id
WHERE photos.id IS NULL

Using includes:

City.includes(:photos).where(photos: {id: nil})

will have the same result, but will result in much uglier SQL like:

SELECT cities.id AS t0_r0, cities.attr1 AS t0_r1, cities.attr2 AS t0_r2, cities.created_at AS t0_r3, cities.updated_at AS t0_r4, photos.id AS t1_r0, photos.city_id AS t1_r1, photos.attr1 AS t1_r2, photos.attr2 AS t1_r3, photos.created_at AS t1_r4, photos.updated_at AS t1_r5
FROM cities LEFT OUTER JOIN photos ON photos.city_id = cities.id
WHERE photos.id IS NULL

When trying to find records with no matching records from the joined table, you need to use a LEFT OUTER JOIN

scope :with_photos, joins('LEFT OUTER JOIN photos ON cities.id = photos.city_id').group('cities.id').having('count(photos.id) > 0')
scope :without_photos, joins('LEFT OUTER JOIN photos ON cities.id = photos.city_id').group('cities.id').having('count(photos.id) = 0')

I used a join to get all the ones with photos:

scope :with_photos, -> { joins(:photos).distinct }

Easier to write and understand, for that particular case. I'm not sure what the efficiency is of doing a join vs doing an includes, though